Extensive single- and two-point measurements have been made in a high-Reynolds- number single-stream mixing layer growing to encounter a wind-tunnel floor on its high-velocity side. The measurements include detailed conditionally sampled results, which separate the turbulent and irrotational contributions to the two-point covariances. These measurements show that the true (vorticity-bearing) large-scale structure in the isolated mixing layer, well away from the region influenced by the floor, is three-dimensional without a trace of the two-dimensional orderly structure found in some two-stream mixing-layer experiments. The structure appears to be a combination of the classical mixing jet and double-roller eddy (Grant 1958); the circulation in the latter is confined almost exclusively to the (z, z)-plane. The large spanwise scales in the potential motion are attributed to the effect of pressure disturbances and not to two-dimensionality of the turbulent structure, as claimed by previous workers. The first effect of the wall is to stretch the streamwise and spanwise scales of the large eddies. Near the high-velocity edge $\overline{u^2}$ is amplified more than $\overline{w^2}$. The surprising result that the low-wavenumber (large-eddy) contribution to $\overline{v^2}$ is amplified across the whole layer is associated with the nature of the mixing jets and the alteration of the pressure field by the wall. The change in turbulence structure occurs before any significant change in the mean-velocity profile, and the implications for the calculation of the change in boundary conditions are discussed. The measurements made after the mixing layer reached the wind tunnel floor will be presented in part 2.